Many modern microprocessors utilize multiple execution units. In many operational situations, not all of these units may be performing work at a given time. For this reason, many designs permit the power to be turned off to these units, while other designs permit them to go to a reduced power state. Other portions of the circuitry, such as certain kinds of memory, may simply consume less power when not processing read or write operations. In other examples various portions of cache and other memories may be turned off when not needed. These techniques may be of use in reducing overall power consumption for battery-powered and other power limited applications.
A problem may arise due to the turning on and off of the various circuit elements. Power supply voltage changes, such as voltage spikes and voltage droops, may be induced by the rate of change of power supply current with respect to time (di/dt). For this reason thermal management strategies that envision dynamically turning off unused circuit elements may give rise to new problems with respect to power supply voltage stability.
Modern chip designs include circuit elements packed tightly together. This increased density produces thermal hot spots. The existence of thermal hot spots is another reason, in addition to the overall conservation mentioned, for the power to be turned off to certain portions of the circuitry. The existence of power supply voltage changes, such as voltage spikes and voltage droops, induced by the rate of change of power supply current with respect to time are even more an issue with these increased density designs.